If you work with CMS Open Data, you quickly learn that “download the ROOT file” is not a trivial step. Large files, long transfers, and fragile connections turn a simple task into an infrastructure problem—often before you even run your first event loop.
Why large CERN ROOT downloads break so often
Most failures are not dramatic. The transfer just stalls, resets, or finishes with a file that exists on disk but is not reliably usable for analysis. The real cost is the time you lose repeating the same transfer attempt, changing tools, and guessing what went wrong.
What we observed: wget / curl resume may be blocked
In our case, the CERN endpoint behaved in a way that effectively prevented resuming interrupted downloads. In practice, this means that after a disconnect, you cannot reliably continue “from the broken point” using typical command-line resume patterns. You restart from zero, and with multi-GB files this quickly becomes a productivity trap.
When a browser is the best tool (yes, seriously)
If you truly need the full file locally and you only need a small number of files, a browser download can be the most practical option. Browsers often handle retries more gracefully, hide some transport complexity, and are simply “sticky” in unstable networks. It’s not elegant, but it is frequently the fastest path to a complete file on disk.
Downside: it does not scale. If you need many files or a reproducible pipeline, manual browser downloads do not survive real workloads.
Why “just use wget/curl” is not always a solution
In an ideal world, command-line tools are perfect for automation. But if the server does not support resume semantics for your request path (or does so inconsistently), then bash scripting becomes a loop of retries that still cannot recover partial progress. This is exactly how teams lose days: the transfer fails at 80–95%, and everything resets.
The scalable approach: extract inside CMSSW, move tables instead of events
For serious work, the more reliable strategy is to avoid downloading huge raw files unless you have a strong reason. Instead:
1) Run the extraction where the environment is already validated (inside CMSSW / the official container).
2) Export analysis-ready outputs (CSV / Parquet / slim ROOT) with a compact schema.
3) Transfer those smaller artifacts, plus reproducible logs and metadata.
This flips the workflow from “move heavy raw data and hope it survives” to “compute close to the data, then move the distilled result.” It is calmer, faster, and easier to verify.
A practical rule of thumb
If you need 1–2 files: a browser download is often the simplest reliable path.
If you need a dataset: treat downloading as part of the pipeline design, not as a side step. Run extraction in CMSSW, export flat tables, and version the outputs with logs/metadata so the result is reproducible.
Conclusion
Large ROOT files from CERN are not “just downloads.” They are a high-friction layer with non-obvious constraints—especially when resume is not reliably supported for CLI tools. The fastest route is often the one that reduces moving parts: either use the browser for a small one-off, or move the computation into the CMSSW environment and only export the final, analysis-ready data.
If you want a calm, reproducible extraction workflow (flat tables, metadata, validation pointers), we can help you design it and deliver ready-to-use outputs.